Klara Soderberg, half of the Swedish sister duo First Aid Kit, started writing lyrics for "Stay Gold" after reading Robert Frost's classic poem of transience, "Nothing Gold Can Stay."

"I had this collection of poetry, and I thought, 'I'll open this and see if there's anything in here that inspires me,' and I came upon the line 'Nothing gold can stay.' That was literally the first thing I saw, and it was perfect."

What with the Frost-inspired song taking top billing on their new album, "Stay Gold," it shouldn't be a surprise that heartbreak, loneliness and homesickness loom large throughout. Klara, 21, and Joanna, 23, are taking a leap into maturity with their third release -- following 2010's "The Big Black & The Blue" and 2012's "The Lion's Roar"-- and they're not looking back.

"On other records, we wrote a lot about nature and fairytale-ish stuff, but if you listen to the lyrics on this one, you can tell it's much more about us, more personal," says Johanna.

A large audience has embraced First Aid Kit's soaring, otherworldly harmonies, but Klara and Joanna don't necessarily want to sound like angels. The duo offers a unique mixture of traditional Americana and Swedish folklore, leaning increasingly hard on Scandinavian melancholy. "The darkness of the winter does bring a kind of sadness with it," says Klara.

In the six years since they launched their careers, the Söderberg sisters have stayed busy, including recording a single for Jack White's Third Man Records, joining Lykke Li in the studio and collaborating with Bright Eyes.

Their success and touring have led them to ponder paths not taken in songs like "Cedar Lane" and "Waitress Song." "It's about dreaming of other lives you could lead," says Klara. "I think everyone has this sort of a secret wish that they could just run away and start a new life somewhere. Not actually wanting to do it, but having a little fantasy about that."

Don't expect Klara and Johanna Soderberg to run away from their lives anytime soon; their music career looks pretty golden.